1. Field of the Invention:
The present invention relates to the preparation of pigmentary red Fe.sub.2 O.sub.3 and, more especially, to the preparation of such oxides by calcination, particularly, of a hydrated ferrous sulfate comprising contaminating amounts of manganese values.
2. Description of the Prior Art:
A wide variety of processes for the production of pigmentary red iron oxides by calcination of ferrous sulfate have long been known to this art. Calcination is an attractive proposition from an economic point of view only if the raw material is inexpensive; it is for this reason that most typically residual ferrous sulfate starting materials are used. These emanate from such process streams as, for example, the sulfuric acid pickling of steel sheets, or from sulfate process titanium dioxide. In both cases, the iron is accompanied by various metal impurities which, unless particular precautions are taken, are transferred into the haematite .alpha.Fe.sub.2 O.sub.3 which is formed during the calcination operation, and impair the color purity of the finished pigment. As a practical matter, manganese is one of the impurities which is most deleterious.
French Pat. No. 744,777 describes a process for producing red iron oxides which are free of foreign cations. That process entails crystallization of the double salt FeSO.sub.4.Na.sub.2 SO.sub.4.4H.sub.2 O. However, the yield of iron does not exceed 40%; 60% of the iron is found in the aqueous effluent.
French Pat. No. 2,438,075 describes a process for separating manganese and most of the metal impurities, thus providing Fe.sub.2 O.sub.3 having excellent pigmentary characteristics. This particular process involves double calcination and an intermediate washing with water, in the course of which the metal impurities and a fraction of the iron values are eliminated. However, the necessity for a second calcination step detrimentally affects the cost of the finished pigment. Furthermore, approximately 10% of the iron is lost in the washing operation which is carried out between the two calcination steps.
U.S. Pat. No. 1,337,402 describes a process wherein the constituents of a stoichiometric mixture of ferrous sulfate and an alkali metal compound, such as sodium carbonate, are interreacted, in the course of crushing them together in the dry state.
The process of the '402 patent thus seeks to prevent the evolution of sulfur oxides during the subsequent calcination operation, and it produces a mixture of iron oxide and alkali metal sulfate. Such process is not economical because of the substantial amount of alkali metal required: 4 gram-atoms of alkali metal per mole of Fe.sub.2 O.sub.3 formed.
U.S. Pat. No. 2,416,138 discloses a process for producing Fe.sub.2 O.sub.3 having a low Mn content, which comprises calcination at a temperature of from 704.degree. to 845.degree. C. (1300.degree. to 1550.degree. F.) of the ferrous sulfate to which an alkali metal salt is optionally added, followed by a washing with water to remove the major amount of the manganese values.
Nonetheless, the '138 patent only discloses the use of sodium chloride, and it is stated that a proportion by weight of 0.3% NaCl/Fe.sub.2 O.sub.3 (namely, 0.008 mole of alkali metal per mole of Fe.sub.2 O.sub.3) is the optimum value and that it is imperative for it to be maintained at levels of less than 3% (namely, 0.08 mole of alkali metal per mole of Fe.sub.2 O.sub.3). Under those conditions, and in order to achieve satisfactory elimination of the manganese, the process must necessarily be carried out at a temperature of less than 845.degree. C. Accordingly, the iron losses in the washing operation are quite substantial, attaining 10%. Moreover, certain grades of iron oxide, which require a calcination temperature of greater than 845.degree. C., cannot be obtained by this process.